(1) Field of the Invention
The present invention relates to a surface acoustic wave (SAW) device comprising an input-side transducer, an output-side transducer, and an electromechanical coupling means provided therebetween so as to change the propagation path of the surface acoustic wave. More particularly, it relates to a SAW device wherein the electromechanical coupling means is formed in a given shape to obtain the desired stopband characteristics.
SAW devices in accordance with the present invention can be used as filtering devices, signal delaying devices, or the like.
(2) Description of the Prior Art
Surface acoustic wave (SAW) devices comprising electromechanical coupling means elements are called multistrip lines (or couplers) which consist of a plurality of electrically conductive strips provided between an input-side transducer and an output-side transducer. These devices, which apply the essential surface acoustic waves of the acoustic waves emitted from the input-side transducer to the output-side transducer and prevent the bulk waves from being applied to the output-side transducer (e.g., U.S. Pat. No. 3,739,290 and U.S. Pat. No. 3,836,876), are already known.
FIG. 1 is a plan view of an example of a prior art SAW device. In FIG. 1, a SAW device 1 comprises a piezoelectric substrate 11 consisting, for example, of lithium niobium oxide (LiNbO.sub.3), an input transducer 16 formed on the surface of the substrate 11, an output transducer 18 formed on the surface of the substrate 11, and the coupler 20 provided between the input transducer 16 and the output transducer 18. The input transducer 16 comprises interdigital electrodes 12 and 14, as shown in FIG. 1. The output transducer 18 is formed in a manner similar to the input transducer 16.
When a high-frequency AC power source is applied to the electrodes 12 and 14 of the input transducer 16, surface acoustic waves W are generated in the surface region of the piezoelectric substrate 11 and propagate in the direction shown in FIG. 1 (the surface acoustic waves propagating in the opposite direction are not shown). The surface acoustic waves W may include essential surface acoustic waves as well as bulk waves, which may exert an adverse influence on device performance such as its filtering characteristics. It is preferable that the bulk waves not be applied to the output transducer 18. The coupler 20 is provided so that only the essential wave component is received. An electric potential is induced at the coupler 20 in response to the received wave. This potential generates surface acoustic waves W', which are applied to the output transducer 18. The coupler 20 includes a plurality of conductive strips 22 formed on the surface of the substrate 11 and parallel to each other, each of which induces a voltage in response to the waveform signal of the essential component of the surface acoustic waves W. The bulk waves Wb cannot otherwise be caught at the strips 22 and will propagate forward to the output transducer. The voltages induced at the conductive strips 22 are transmitted to the opposite ends of the strips, where the strips are associated with the substrate 11 so as to generate secondary surface acoustic waves W'. The output transducer 18 receives the secondary surface acoustic wave W' and outputs an electrical signal in response to the received wave W'. As mentioned above, the direction of propagation is changed by the coupler 20 as shown and only the essential surface component of the wave is applied to the output transducer 18. This gives the required characteristics, such as a signal delaying feature, between the input transducer 16 and the output transducer 18.
The coupler 20 mentioned above has a considerably wide frequency bandwidth. Therefore, prior art SAW devices have such drawbacks as an inability to obtain given filtering characteristics. This is the case, for example, with filters used in television sets, which have a deep trap level at both ends of the passband.